How to get http params from Cowboy?

You should use the cowboy_http_req:qs_val/2 function, e.g. cowboy_http_req:qs_val(<<"a">>, Req), look at https://github.com/extend/cowboy/blob/master/examples/echo_get/src/toppage_handler.erl for an example.

You can also use cowboy_http_req:qs_vals/1 to retrieve a list of all query string values.